Vascular calcification, commonly known as hardening of the arteries, is a major risk factor and independent predictor of cardiovascular mortality in the general population as well as high-risk diabetic and chronic kidney disease patients. Vascular calcification is highly correlated with atherosclerotic plaque burden, and the degree of calcification predicts increased risk of adverse clinical events and death. In addition, vascular calcification is associated with hypertension, cardiac valve disease, artificial heart valve calcification, and other forms of arteriosclerosis. It leads to cardiac valve stenosis, increased pulse pressure, hypertension and may contribute to plaque rupture, all of which can lead to heart failure. Importantly, there are currently no drugs to treat vascular calcification, and drugs that are commonly used to treat cardiovascular disease, such as statins, are not effective against vascular calcification. Thus, there is a great need for a better understanding of the origins of cells that participate in vascular calcification, as well as the mechanisms that regulate these cells such that appropriate preventative and therapeutic strategies can be developed to treat this debilitating pathology. In the past funding period, we identified a critical role for vascular smooth muscle cell (SMC) lineage reprogramming to osteochondrogenic precursors in vascular calcification. Using genetic fate mapping strategies in mouse models of arterial intimal (LDLR-/- and ApoE-/- mice) and medial calcification (MGP-/- mice), SMCs were found to be a major source of osteochondrogenic precursors and chondrocytes in the calcified vasculature. In both intimal and medial calcification models, the lineage reprogramming of SMCs towards an osteochondrogenic was preceded by de novo expression of Runx2 (also known as Cbfa1), a transcription factor required for normal bone and cartilage development. Furthermore, Erk1/2 signaling was required for SMC osteochondrocytic phenotype change in vitro. Based on these data, our overall hypothesis is that SMCs undergo lineage reprogramming in response to disease-specific, procalcific cues that convergeon a common downstream mediator, Runx2 (Figure 1) via the activation of Erk1/2. Runx2 phosphorylation by Erk1/2 leads to turn on of osteochondrogenic gene expression, and turn off of smooth muscle gene expression thereby reprogramming the smooth muscle cell towards an osteochondrogenic fate. In this proposal, we will definitively test this hypothesis by determining whether Runx2/Cbfa1 is required for vascular calcification under different disease settings and by further delineating the requirement and mechanisms of Erk signaling in Runx2- associated SMC lineage reprogramming and calcification.